Planet Hulk is a bit of a ridiculous concept, and would probably cost way more than Marvel wants to spend on a Hulk solo film right now. They're not quite to that point with The Hulk, and may never be in this Marvel film universe.

"Hulk smash puny human who say Planet Hulk ridiculous"
Judging from the reaction of the "Avengers" movie, The good ol green guy just got a boost, and is big-budget worthy

What would be better than Hulk as a gladiator, taking on creatures as powerful as he is, taking on a wife, and becoming ruler of that world, The movie starts with Hulk as the ruler in the first 20 minutes, but then goes back to the point where he was banished, and then forced to fight, and so on, and this all happens OUTSIDE of the current MCU!